usb: dwc2: Move phy init into core

As the phy initialization is almost the same in host and gadget
mode. This only move the phy initialization functions into core.c
for now, the goal is to share theses functions between the two modes.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Jules Maselbas <jmaselbas@kalray.eu>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>